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Full text of "Text Book Of Mechanical Engineering"

126                          Tempering Colours.

first of these is that of hardening. Here the steel is heated as
equally as possible to a c cherry red/ and not more; and on with-
drawing from the fire it is plunged vertically into a vessel of cold
water. The quickness of cooling has a great effect on the hard-
ness, and this may be accelerated by moving the article about in
the water. Cracking or warping will also be prevented by
judicious motion.

The steel is now so hard that it will scratch glass. It must
next be tempered or let down to the required degree of hardness.
If the tool be again heated to cherry red, and allowed to cool
slowly it will by that means have become annealed, and will be
at its softest; but if it only be heated to one of the temperatures
in the following table (Fig. 117^, Plate III.), and then cooled
rapidly, it will take a particular degree of hardness corresponding
to that temperature, and to be obtained at no other. When
letting-down, the softest tool will be that which is cooled at the
highest temperature, and the hardest that coaled at the lowest
temperature.

The exact temperature which the tool has assumed is ascer-
tained by the colour which appears on the brightened surface, due
to a film of oxide of iron formed by contact with the air. There
is some difference of opinion as to the requisite hardness for
certain purposes, and slightly different colours are required for
different steels, but Plate III. is suitable for average tool steel.

Tempering a Chisel or Drill.—To make the matter
clearer we will take the case of a chisel for chipping metal. It is
forged out of a steel bar of the section shewn at A, Tig. 122, and
is drawn out (at as low a heat as possible, to prevent burning) to
a flat point as at B. This point is now to be hardened and
tempered, while the rest of the chisel is to remain in its natural
condition. Whenever the tempering is accomplished by quench-
ing in water, the preliminary process of hardening must always be
performed, otherwise the tempering would have no effect. In the
case of the chisel, or any too! having a point requiring a particular
temper, tbe two processes are performed at one heat, but it must
be quite clear that hardening is not therefore dispensed with.
Heating the whole chisel to a cherry red, the part a b only is
quenched in water, and so becomes very hard* Now rub Jthe
point of the chisel with a stone to brighten it a little, and, as the